Re: [PATCH 5/9] fs: remove various compat readv/writev helpers

Christoph Hellwig
2020-09-23T14:32:57Z

On Wed, Sep 23, 2020 at 03:25:49PM +0100, Al Viro wrote:
> On Wed, Sep 23, 2020 at 08:05:43AM +0200, Christoph Hellwig wrote:
> > COMPAT_SYSCALL_DEFINE3(readv, compat_ulong_t, fd,
> > - const struct compat_iovec __user *,vec,
> > + const struct iovec __user *, vec,
>
> Um... Will it even compile?
>
> > #ifdef __ARCH_WANT_COMPAT_SYS_PREADV64
> > COMPAT_SYSCALL_DEFINE4(preadv64, unsigned long, fd,
> > - const struct compat_iovec __user *,vec,
> > + const struct iovec __user *, vec,
>
> Ditto. Look into include/linux/compat.h and you'll see
>
> asmlinkage long compat_sys_preadv64(unsigned long fd,
> const struct compat_iovec __user *vec,
> unsigned long vlen, loff_t pos);
>
> How does that manage to avoid the compiler screaming bloody
> murder?

That's a very good question. But it does not just compile but actually
works. Probably because all the syscall wrappers mean that we don't
actually generate the normal names. I just tried this:

--- a/include/linux/syscalls.h
+++ b/include/linux/syscalls.h
@@ -468,7 +468,7 @@ asmlinkage long sys_lseek(unsigned int fd, off_t offset,
asmlinkage long sys_read(unsigned int fd, char __user *buf, size_t count);
asmlinkage long sys_write(unsigned int fd, const char __user *buf,
size_t count);
-asmlinkage long sys_readv(unsigned long fd,
+asmlinkage long sys_readv(void *fd,

for fun, and the compiler doesn't care either..